1. Field of Use
The present invention relates to data processing systems and more particularly to a system which facilitates memory access.
2. Prior Art
In current data processing systems, it is necessary to provide storage for important control routines as for example, initialize and start-up routines. Moreover, it is important to ensure that the control routines remain intact and are only accessed by certain routines.
One way of providing the above protection in some systems is to store important control routines nonalterable storage and to include checks to ensure detection of illegal accesses.
Normally, the permanent storage is included as either the upper or lower addressing range of a read/write memory. Stated differently, the permanent storage is included as a part of the read/write memory.
It has been found that the routines included in the permanent storage would be run within a fixed address range corresponding to the extent of permanent storage. When it is required to have a routine access information outside the permanent storage area such as to process an exception condition or interrupt, it is necessary to provide additional registers which are preconditioned to indicate the source of the exception condition. The system is then required to execute a common routine for processing the exception, and then branches to a predetermined address specified by another register which would process the exception condition. Following such processing, the system returns control to the main routine via a return instruction sequence.
In the above arrangement, it is necessary to provide appropriate checking circuits to insure that all accesses made to the control routines are authorized. Additionally, it is also necessary to provide for offsetting the address values of instructions included within the routines in order to ensure that accesses are made only to the proper areas of memory.
The significant disadvantages of the above arrangements are the lack of flexibility in accessing the different types of memories and the attendant inefficiencies resulting in switching between the memories. Another disadvantage is the requirement for additional checking circuits for signaling improper accesses.
Accordingly, it is a primary object of the present invention to provide a system for facilitating accessing information from a number of memory units.
It is a more specific object of the present invention to provide a system which enables access to a number of memories in a manner which ensures detection of improper memory accesses.